The present invention relates to a carbon material which intercalates into or deintercalates from lithium, and to a method for manufacturing the same. In particular, the present invention relates to a lithium secondary battery, which uses carbon material as a negative electrode active material, having a high energy density and a long life. The lithium battery is suitable for use in portable apparatus, electric automobiles, power storage, etc.
The lithium secondary battery using lithium metal for the negative electrode has some problems relating to safety. For example, lithium easily deposits like dendrite on the lithium metal negative electrode during repeated charging and discharging of the battery, and if the dendritic lithium grows to a positive electrode, an internal short circuit will be caused between the positive electrode and the negative electrode.
Therefore, a carbon material has been proposed as the negative electrode active material in place of lithium metal. Charge and discharge reactions involve lithium ion intercalation into the carbon material and deintercalation from the carbon material, and so lithium is hardly deposited like dendrite. As for the carbon material, graphite is disclosed in JP-B-62-23433 (1987).
The graphite disclosed in JP-B-62-23433 (1987) forms an intercalation compound with lithium, because of intercalation or deintercalation of lithium. Thus, graphite is used as a material for the negative electrode of the lithium secondary battery. In order to use graphite as the negative active material, it is necessary to pulverize the graphite to increase the surface area of the active material, which constitutes a charge and discharge reaction field, so as to allow the charging and discharging reactions to proceed smoothly. Desirably, it is necessary to pulverize the graphite to powder having a particle diameter equal to or less than 100 xcexcm. However, as is apparent from the fact that graphite is used as a lubricating material, the graphite easily transfers its layers. Therefore, its crystal structure is changed by the pulverizing process, and formation of the lithium intercalated compound might be influenced by undesirable effects of the pulverizing process. Accordingly, the graphite after the pulverizing process has a great deal of crystalline structural defects. In a case when graphite is used as an active material for the negative electrode of the lithium secondary battery, a disadvantage results in that a large capacity can not be obtained. Furthermore, preferable performances of rapid charging and discharging are not obtained because the lithium intercalation-deintercalation reaction is disturbed by the above defects.
The object of the present invention is to solve the above problems, to provide a carbon material having a large lithium intercalation-deintercalation capacity and a method for manufacturing the same, and to provide a non-aqueous secondary battery which has a large capacity and is superior in its rapid charging and discharging characteristics using the above-mentioned materials.
The crystalline structure of the graphite powder relating to the present invention has a feature that an existing fraction of the rhombohedral structure in the crystalline structure of the graphite is small (equal to or less than 20%). Another feature is that an existing fraction of the hexagonal structure is great (at least 80%). The above existing fractions of the rhombohedral structure and the hexagonal structure can be determined by analyzing the intensity ratio of the peaks in X-ray diffraction of the material.
The graphite powder relating to the present invention is manufactured by a method comprising the steps of graphitizing treatment (heating at least 2000xc2x0 C.) of raw material such as oil cokes and coal cokes, pulverizing the graphitized raw material to powder, sieving the powder for obtaining the maximum particle diameter equal to or less than 100 xcexcm, heating the powder to at least 900xc2x0 C. as a heat treatment, and further heating the powder to at least 2700xc2x0 C. for eliminating impurities such as Si. For instance, when the powder is heated to at least 2700xc2x0 C., Si, which is a main component of the impurities, can be reduced to less than 10 ppm. The heat treatment of the powder for eliminating impurities can be omitted depending on the content of the impurities in the raw material. In the pulverizing process, various conventional pulverizers can be used. However, a jet mill is preferable, because pulverization with the jet mill generates the minimum destruction of the graphite crystalline structure in the raw material.
Furthermore, the graphite powder relating to the present invention can be obtained by immersing into an acidic solution containing at least one compound selected from a group consisting of sulfuric acid, nitric acid, perchloric acid, phosphoric acid, and fluoric acid as an immersing treatment, after pulverizing the raw graphite to obtain graphite powder having a particle diameter equal to or less than 100 xcexcm, subsequently washing with water, neutralizing, and drying.
The non-aqueous secondary battery for achieving the object of the present invention can be manufactured by using the graphite powder relating to the present invention as the negative electrode active material, and the positive electrode is desirably composed of a material comprising a compound expressed by a chemical formula of LixMO2 (where; X is in a range from zero to 1, and M is at least any one of chemical elements selected from a group of Co, Ni, Mn and Fe), or LiMn2O4, that is a lithium transient metal complex oxide.
The active materials for the battery are generally used in the form of a powder in order to facilitate the charging and discharging reaction by increasing the surface area of the active material, which constitutes a reaction field of the charging and discharging reaction. Therefore, the smaller the particle size of the powder is, the more will performance of the battery be improved. Furthermore, when the electrode is manufactured by applying an agent mixed with the active material and a binding agent to a current collector, the particle diameter of the active material is desirably equal to or less than 100 xcexcm in view of applicability and maintaining preciseness of thickness of the electrode.
As for the negative electrode active material for the lithium secondary battery, natural graphite, artificial graphite, and others are disclosed. However, for the above described reason, it is necessary to pulverize these materials. Therefore, in the pulverizing process, various graphite powders having a is diameter equal to or less than 100 xcexcm were prepared with various pulverizing methods using a ball mill, a jet mill, a colloidal mill and other apparatus, for various times. And, the lithium intercalation-deintercalation capacity of the various graphite powders were determined for determining a superior material for the negative electrode material of the lithium secondary battery.
However, the graphite powder obtained by the above method had lithium intercalation-deintercalation amounts per weight in a range of 200-250 mAh/g, and their capacities as the material for the negative electrode of the lithium secondary battery were not enough.
In order to investigate the reason for the small capacity, crystalline structures of the above various graphite samples were determined by an X-ray diffraction method. FIG. 1 indicates an example of the results. Four peaks can be observed in a range of the diffraction angle (2xcex8, xcex8: Bragg angle) from 40 degrees to 50 degrees in the X-ray diffraction pattern. The peaks at approximately 42.3 degrees and 44.4 degrees are diffraction patterns of the (100) plane and the (101) plane of hexagonal structure of the graphite, respectively. The peaks at approximately 43.3 degrees and 46.0 degrees are diffraction patterns of the (101) plane and the (102) plane of the rhombohedral structure of the graphite, respectively. As explained above, it was apparent that there were two kinds of crystalline structure in the pulverized graphite.
Further, the existing fraction (X) of the rhombohedral structure in the graphite powder was calculated by the following equation (Equation 1) based on the data of the observed peak intensity (P1) of the (100) plane of the hexagonal structure, the observed peak intensity (P2) of the (101) plane of the rhombohedral structure, and a theoretical relationship of the intensity ratio in the X-ray pattern of graphite. As a result, it was revealed that the graphite having the rhombohedral structure was contained by approximately 30% in all the graphite pulverized equal to or less than 100 xcexcm in particle diameter.
xe2x80x83X=3P2/(11P1+3P2) xe2x80x83xe2x80x83(Equation 1)
Similarly, the existing fraction (X) of the rhombohedral structure of the graphite powder was verified by the relationship of the observed peak intensity (P1) of the (100) plane of the hexagonal structure, the observed peak intensity (P3) of the (102) plane of the rhombohedral structure, and the theoretical relationship of the intensity ratio in the X-ray pattern of the graphite. In this case, the following equation 2 was used instead of the equation 1. As a result, it was confirmed that graphite having the rhombohedral structure was contained by approximately 30% in all the graphite pulverized equal to or less than 100 xcexcm in particle diameter.
X=P3/(3P1+P3)xe2x80x83xe2x80x83(Equation 2)
The reason for existence of the two kinds of crystalline structure is assumed to be that the graphite itself has a lubricating property, and the original graphite having a hexagonal structure transforms to graphite having rhombohedral structure by the pulverizing process with strong shocks. Graphite powder of a few microns in particle diameter obtained by further continued pulverization had a significantly broadened X-ray diffraction peak (P4) at the (101) plane of the hexagonal structure, and it was revealed that the content of amorphous carbon in the graphite was increased because the half band width of the peak was increased. Accordingly, the reason for the small lithium intercalation-deintercalation capacity of the conventional graphite powder can be assumed to be due to the fact that the crystalline structure of the graphite has been transformed to the rhombohedral structure and has generated the amorphous carbon, with the result that the lithium intercalation-deintercalation reaction is disturbed by the rhombohedral structure and the amorphous carbon.
Analysis of the impurities of the graphite powder revealed that impurities such as Si, Fe, and others were present in an amount more than 1000 ppm. Naturally, in addition to the impurities contained in the raw material, impurities from a processing apparatus, such as a ball mill, a jet mill, and the like, can be mixed into the graphite during the pulverizing process. Therefore, the influence of the above impurities can be assumed as another reason for the small capacity, in addition to the above formation of the rhombohedral structure and amorphous carbon.
In accordance with the present invention, a graphite powder having a particle diameter equal to or less than 100 xcexcm, wherein the content of the above described rhombohedral structure is less than 30% and the content of the amorphous carbon is small, has been developed. Additionally, the content of Si in particular, which is the main component of the impurities in the graphite powder, has been decreased to an amount equal to or less than 10 ppm. Therefore, extremely high purity is one of the features of the graphite relating to the present invention. The particle diameter equal to or less than 100 xcexcm is determined with an intention to use the graphite for a battery, as described previously. Therefore, when the graphite of the present invention is used for other purposes, the particle diameter of the graphite is not necessarily restricted to a size equal to or less than 100 xcexcm.
Hereinafter, details of the graphite powder relating to the present invention, and the method for manufacturing the same will be explained.
Two methods (manufacturing method 1 and manufacturing method 2) for obtaining graphite having a small fraction of the rhombohedral structure are disclosed.
(Manufacturing Method 1)
As for raw material (raw graphite) for the graphite powder of the present invention, both natural graphite and artificial graphite can be used. In particular, flaky natural graphite is preferable. Among the raw graphite, the one having a maximum diffraction peak in the X-ray diffraction pattern by the CuKxcex1 line which appears at a diffraction angle (2xcex8, xcex8: Bragg angle) in a range from 26.2 degrees to 26.5 degrees, that is, where an interval between two graphite layers is equal to or less than 0.34 nm, is desirable. As a result, a graphite powder containing a small amount of the rhombohedral structure can be obtained from the high crystalline raw material.
As for the pulverizing apparatus for crushing the raw graphite to a particle diameter equal to or less than 100 xcexcm, a jet mill is desirable. The reason is that the amorphous carbon is generated less with the jet mill than in the case when another pulverizing apparatus is used.
The pulverized raw graphite (raw powder) contains graphite having a rhombohedral structure by approximately 30% as previously described. Then, in accordance with the present manufacturing method 1, the existing fraction of the rhombohedral structure is decreased by the following heat treatment.
The heat treatment is performed to at least 900xc2x0 C. under an inert gas atmosphere. As for the inert gas, nitrogen gas, argon gas, and the like is used. The inert gas atmosphere can also be maintained by covering the raw powder with cokes to seal it from the atmosphere.
The heat treatment is the most important process in the present invention for transforming the rhombohedral structure to a hexagonal structure. It is necessary to perform the heat treatment after pulverization of the raw graphite (more preferably, at the last stage of the graphite powder manufacturing process of the present invention).
If the heat treatment is performed before the pulverization of the graphite and subsequently the graphite is pulverized, graphite powder containing a rhombohedral structure in a quantity as small as possible, which is the object of the present invention, can not be obtained. The graphite powder containing the rhombohedral structure graphite in a quantity as small as possible can be obtained only by employing the heat treatment after the pulverizing process (more preferably, at the last stage of the graphite powder manufacturing process of the present invention) as the present invention proposes.
The raw graphite powder contains Al, Ca, Fe, and particularly a large amount of Si, as impurities. The impurities can be eliminated by heating and sublimating the materials to at least 2700xc2x0 C. Therefore, the heating temperature in the heat treatment is preferably at least 2700xc2x0 C. in order to perform a purification treatment concurrently.
(Manufacturing Method 2)
The raw graphite and the pulverizing process is the same as the above manufacturing method 1.
The graphite powder of the present invention can be obtained by treating the graphite powder obtained by the pulverizing process with an acidic solution containing at least one compound selected from a group consisting of sulfuric acid, nitric acid, perchloric acid, phosphoric acid, and fluoric acid, and subsequently washing with water, neutralizing, and drying. During the treatment, a compound is formed with anions in the above acidic solution and the graphite, and the rhombohedral structure graphite is eliminated by the formation of the compound. The anions from the acidic solution in the compound are eliminated from the compound during the washing, the neutralizing, and the drying, and the graphite powder relating to the present invention can be obtained.
The crystalline structure of the graphite powder of the present invention obtained by the above manufacturing methods 1 and 2 was analyzed by X-ray diffraction. The ratio of P1 and P2, (P2/P1), was less than 0.92, and the half band width of P4 was less than 0.45 degrees. The ratio of P1 and P3, (P3/P1), was less than 0.75.
By substituting the above observed data for the equations 1 and 2, the fact that the existing fraction of the rhombohedral structure has been decreased to less than 20% and the existing fraction of the hexagonal structure has been increased at least 80% was confirmed. Simultaneously, the content of Si was confirmed to be less than 10 ppm from the result of impurity analysis.
Then, an electrode was prepared using the graphite powder of the present invention as an active material, and the lithium intercalation-deintercalation is capacity was studied. As a result, the lithium intercalation-deintercalation capacity of the graphite powder of the present invention was 320-360 mAh/g per unit weight of the active material, and the capacity was significantly improved in comparison with the capacity of the conventional graphite material (200-250 mAh/g). Furthermore, it was found that the preferable existing fraction of the rhombohedral structure was equal to or less than 10%, because the less the existing fraction of the rhombohedral structure in the graphite powder of the present invention is, the more will the capacity be increased. Where the fraction of the rhombohedral structure is 10% or less, and from Equations 91) and 92) herein, respectively, P2/P1 is 0.41 or less and P3/P1 is 0.33 or less.
Accordingly, the rhombohedral structure is evidently a crystalline structure which hardly will intercalate or deintercalate lithium. Therefore, it is assumed that the high lithium intercalation-deintercalation capacity of the graphite powder of the present invention is achieved by especially decreasing the existing fraction of the rhombohedral structure and increasing the existing fraction of the hexagonal structure.
The feature of the lithium secondary battery of the present invention is in using the graphite powder of the present invention as the negative active material. The lithium secondary battery relating to the present invention has a large load capacity, and a high energy density can be realized.
As a result of an evaluation of the characteristics of the lithium secondary battery of the present invention, it was confirmed that the lithium secondary battery of the present invention had a superior performance in rapid charging and discharging characteristics, and a decreasing ratio of the capacity was improved at least 30% in comparison with the conventional lithium battery under a same rapid charging and discharging condition. The reason for the improvement can be assumed to relate to the fact that the reversibility for the lithium intercalation-deintercalation reaction of the graphite of the present invention is improved in comparison with the conventional carbon material by decreasing the existing fraction of the rhombohedral structure and eliminating the influence of the impurities, such as Si.
As the positive active material for the lithium secondary battery of the present invention, materials such as LixCoO2, LixNiO2, LixMn2O4, (where, X is in a range 0-1) and the like are desirable because a high discharge voltage of at least 3.5 V can be obtained, and the reversibility of the charging and discharging of the positive electrode itself is superior.
As for the electrolytic solution, a mixed solvent composed of ethylene carbonate mixed with any one selected from a group consisting of dimethoxyethane, diethylcarbonate, dimethylcarbonate, methylethylcarbonate, xcex3-butylolactone, methyl propionate, and ethyl propionate, and at least one of the electrolytes selected from a group consisting of salts containing lithium such as LiClO4, LiPF6, LiBF4, LiCF3SO3, and the like are used. It is desirable to adjust the lithium concentration in a range 0.5-2 mol/l, because the electric conductivity of the electrolytic solution will be favorably large.